Revert size to be unsigned for ArrayRef

Revert size to be unsigned for ArrayRef

Given the decision for std::span to be signed this became
inconsistent with future C++ while already being inconsistent
with std::vector. The goal of the original change, to have
signed variables for all arithmetic including loop indices,
can be achieve by using ssize everwhere arithmethic is used.
ssize is both available as free function (also for std::vector)
and member function.

Related #2826

Change-Id: Icc5d0f9561c610cbce34c575d268171e9890ca9c